FR2917164A1 - METHOD OF ESTIMATING THE PERFORMANCE OF A TIRE IN A BRAKING SITUATION - Google Patents

Abstract

The invention relates to a method for estimating the performance, in a braking situation, of a tire capable of equipping a vehicle, this method comprising an experimental phase (EXPERIM) including at least one evaluation step (EVAL_MU) the coefficient of adhesion (& mu) of the tire on the ground.According to the invention, this evaluation step is performed for different values (Vi) of the relative speed of movement of the tire axis relative to the ground and for different values (gi%) of the slip rate of the tire on the ground, and the experimental phase (EXPERIM) further comprises a modeling step (MODEL) of representing the coefficient of adhesion of this tire, as it results the evaluation step (EVAL_MU), a function (MU3D) of the relative speed of movement of the tire axis relative to the ground and the rate of slip of the tire on the ground.

Description

METHOD OF ESTIMATING THE PERFORMANCE OF A TIRE IN A SITUATION OF

BRAKING.

  The present invention relates, in general, the equipment of land vehicles in tires. More specifically, the invention relates to a method for estimating the performance, in a braking situation, of a tire capable of equipping a vehicle, this method comprising an experimental phase including at least one step of evaluating the coefficient of grip of the tire on the ground.

  Although braking performance of tires has in the past often been evaluated by braking distance tests of a vehicle equipped with these tires, these performances are now more commonly evaluated on the basis of measurements of the braking distance of a vehicle equipped with these tires. coefficient of adhesion of a single tire fitted to a trailer, these measurements being correlated with measurements of the slip rate of this tire and with indicators derived from these measurements.

  Although this technique has the advantage of only requiring the use of a single tire, it often leads to insufficient reliability evaluations, and does not include ensuring that several different tires, classified in increasing order of performance based on these tests, develop, once mounted on the same type of vehicle, performance that rank them in the same order.

  In this context, the purpose of the invention is to propose a method for estimating the performance of a tire in

  braking situation which, in spite of its simplicity, has an increased reliability compared to known techniques.

  To this end, the method of the invention, moreover in accordance with the preamble above, is essentially characterized in that the step of evaluating the coefficient of adhesion of the tire on the ground is carried out for different values of the speed of relative displacement of the tire axis with respect to the ground and for different values of the slip rate of the tire on the ground, and in that said experimental phase further comprises a modeling step of representing the coefficient of adhesion of the tire. this tire, as a result of the evaluation step, a function of the relative speed of displacement of the tire axis relative to the ground and the rate of sliding of the tire on the ground.

  Preferably, the step of evaluating the coefficient of adherence of the tire comprises the simultaneous operations of rolling the tire on a reference surface similar to the ground, applying the tire to the reference surface with a bearing force of known intensity by adjustment and / or measurement, to scroll, relative to the axis of the tire, the reference surface at an instantaneous speed of relative movement, known by adjustment and / or by measurement, and decreasing from a a maximum value up to a minimum value, to measure the instantaneous angular speed of rotation of the tire, and periodically applying to the tire a braking torque of known instantaneous value by adjustment and / or by measurement and capable of causing a sliding of the tire on the reference surface.

  The coefficient of adhesion can typically be evaluated by the ratio, to the bearing force, of the longitudinal force corresponding to the braking torque.

  The operation of periodically applying the braking torque is advantageously regulated so as to vary the slip rate of the tire relative to the reference surface between a minimum value preferably zero and a maximum value at least equal to 20. It is also possible to to provide that the operation of periodic application of the braking torque is regulated to reach periodically, or even exceed, the value of the slip rate for which the value of adhesion is maximum. In any case, the step of evaluating the coefficient of adherence of the tire may advantageously be implemented in a room with a substantially constant temperature.

  The modeling step comprises for example an optimization operation consisting in searching for the values to be assigned to predetermined parameters so that a parametric expression of said function of the relative speed of displacement of the tire axis with respect to the ground and the slip rate of this tire on the ground best corresponds to the coefficient of adhesion as evaluated, during the evaluation step, for the different values of the relative speed of movement of the tire axis relative to the on the ground and for the different values of the slip rate of the tire on the ground. In this case, the optimization operation may use the following parametric expression: Mu3D = (D * sin (C * atan (B * X1-E * (B * X1-atan (B * X1)))) ) + Sv, where D, B and Xl correspond to the following parametric expressions: D = DO + D1 * V + D2 * V2, B = BO + B1 * V + B2 * V2, 10 X1 = (g% / 100) + Sh, and where V is the instantaneous speed of relative displacement of the tire axis relative to the reference surface, and where g% is the slip rate of that tire relative to

  15 at this surface, defined by: g% = 100 * (V - Vp) / V, where Vp is the peripheral speed of the tire.

  The method of the invention may also include a predictive phase including an operation of determining an overall tire performance indicator, consisting of integrating, with respect to the instantaneous speed of relative displacement of the tire axis and the ground, and with respect to the sliding rate of this tire on the ground, the function representative of the coefficient of adhesion of the tire, over a range of instantaneous speeds and slip rates corresponding to a braking sequence

  30 real or simulated. In this case, the braking sequence chosen to define the range of integration of the instantaneous speed and slip rate preferably includes a modulation of the braking torque of the type found on a vehicle equipped with an anti-lock braking system. wheels commonly called ABS. Other features and advantages of the invention will emerge clearly from the description which is given below, by way of indication and in no way limiting, with reference to the appended drawings, in which: FIG. 1 is a diagram of operational organization A method according to a possible embodiment of the invention; FIG. 2 is a schematic perspective view of an installation that can be used to implement the method of the invention; FIG. 3 is a diagram representing in ordinate, as a function of time for example expressed in hundredths of a second and taken as the abscissa, a hundredfold of the value of the coefficient of adhesion (dashed curve), the peripheral speed of the tire ( mixed line curve) and the speed of instantaneous displacement of the tire axis relative to the reference surface (solid curve), in a possible embodiment of the method

  Of the invention; and FIG. 4 is a diagram illustrating a representation, according to the invention, of a coefficient of adhesion in the form of a function of a speed and

  30 slip rate. As previously announced, the invention relates to a method for estimating the performance, in a braking situation, of a tire PN intended to equip a

35 vehicle.

  Traditionally, such a method comprises an experimental phase EXPERIM (Figure 1) of which an EVAL MU stage is devoted to the evaluation of the coefficient of adhesion of the tire on the ground.

  It is recalled that the coefficient of adhesion of a tire on the ground is represented and therefore evaluated by the ratio Fx / Fz, where Fx is the longitudinal braking force corresponding to a braking torque applied by the tire between its axis and the ground, and where Fz is the load of the tire, that is to say the force of support it transmits to the ground. According to a first aspect of the invention, the evaluation step EVAL MU is performed for different values, denoted Vi, of the speed V of relative displacement of the X axis of the tire relative to the ground, and for different values, noted gi%, the slip rate g% of the tire on the ground, so as to obtain so many values, noted i, the coefficient of adhesion g. It is recalled on this occasion that the slip rate g% of a tire is defined by: g% = 100 * (V - Vp) / V, where Vp is the speed of the periphery of the tire. FIG. 2 very schematically illustrates an installation that can be used, in no way limiting, for the implementation of the evaluation step EVAL MU. In the illustrated example, this installation comprises an endless belt stretched between two rollers, one of which is rotated by a motor MOT, the endless belt being covered with an adhered material which allows it to simulate the soil S a dry runway, or a wet runway depending on the desired result. The PN tire to be characterized is inflated to a predefined operating pressure and mounted on a CH yoke which applies it on the reference surface S of the endless belt with a predefined resting force Fz. The PN tire is for example mounted on a MYD torque hub and / or rotatably coupled to a coding wheel (not shown), this equipment making it possible to have, in addition to the quantities Fz and V, a signal representative of the angular velocity instantaneous rotation of the tire PN around its axis X, and a signal representative of the longitudinal force Fx corresponding to a braking torque K applied to the tire PN by means of a brake FR whose engine is linked to CH screed.

  As is also known to those skilled in the art, the peripheral speed Vp of the tire, also called the tangential speed, is related to the angular rotation speed w by the effective rolling radius Reff, this radius being able for example to be obtained as being equal to the value taken by the ratio V / w during the rolling phases without braking. The PN tire is disposed parallel to the median axis of the endless belt and perpendicular to the reference surface S, that is to say with a zero angle of camber and a camber angle. This installation, which is very advantageously placed in a room with a substantially constant temperature, makes it possible to control the variables that constitute, on the one hand, the speed V of travel of the surface S with respect to the fixed axis X of the tire PN, and of on the other hand the slip rate g%. By definition of the sliding ratio, the latter takes the value 0% when Vp = V, that is to say when the tire PN is in pure rolling on the surface S, and takes the value 100% when Vp = 0 that is, when the wheel fitted with the PN tire is totally blocked by braking. After a phase of heating the PN tire by rolling at an initial speed, the evaluation step EVAL_MU continues with a braking cycle, during which a braking torque K is periodically applied to the tire PN for a speed V decreasing

  15 while the instantaneous variables V, o, Fz and Fx are detected, this step makes it possible to obtain curves such as those illustrated in FIG. of the value of the coefficient of adhesion the curve in phantom represents the peripheral speed Vp of the tire, and the curve in solid line represents the instantaneous speed of relative displacement of the X axis of the tire with respect to the reference surface S . This FIG. 3 illustrates a braking sequence SQC FR that is relevant for characterizing a tire in a current vehicle braking situation, namely a sequence of the type that is generated by an ABS-type braking system, ie say anti-lock wheels, during a braking test of 100km / h at 0km / h. In such a sequence, the speed V decreases monotonically, ideally linear, from a maximum speed VO to a minimum speed of preferably zero, in a configurable time interval of between 3 and 6 seconds. In this interval, it is possible to carry out 5 to 10 cycles of variation of the sliding ratio g% between a zero value and a maximum value g% max for example

  configurable between 20% and 100%. Under these conditions, the aforementioned parameter g% max is chosen so as to reach and slightly exceed the value of the slip rate for which the longitudinal braking force Fx is maximum. According to a second aspect of the invention, the EXPERIM experimental phase comprises a modeling step MODEL which consists of representing the adhesion coefficient g of this tire, as manifested by its multiple values i obtained during the evaluation step EVALMU, by a function, denoted MU3D, of the

  Velocity V and slip rate g%. To do this, the modeling step MODEL preferably comprises an optimizing operation OPTIM which consists in searching for the values to be assigned to predetermined parameters so that a parametric expression of the MU3D function mentioned above best corresponds to the coefficient d adhesion g as evaluated in the EVAL MU evaluation step. For example, the parametric expression of the MU3D function may take the form: Mu3D = (D * sin (C * atan (B * X1-E * (B * X1-atan (B * X1))))) + Sv, where D, B and X1 themselves correspond to the following parametric expressions: D = DO + D1 * V + D2 * V2,

  B = BO + B1 * V + B2 * V2, X1 = (g% / 100) + Sh,

  where Sv is for example equal to 0.01, and where Sh can be zero.

  The optimization operation OPTIM, which can use any known optimization tool, therefore consists in finding the values to be assigned to the parameters BO, B1, B2, C, DO, D1, D2, and E at least, so that the The parametric expression MU3D above takes, for a set as large as necessary, different values V 1 of the speed V and different values g 1% of the sliding ratio g%, respective values which are as close as possible to the corresponding values i that were recorded for the coefficient of adhesion during the EVAL MU evaluation step. _

  In practice, it may be desirable to carry out, prior to the OPTIM operation, a filtering operation on the measurement signals such as w, V, Fz and Fx to overcome any artefacts related to measurement noise. .

  Once the values of the parameters BO, Bi, B2, C, DO, D1, D2, E, Sh and Sv have been determined, the function MU3D representative of the coefficient of adhesion can be plotted in a three-dimensional space as illustrated in FIG. 4 and of which a first dimension corresponds to the slip rate g%, a second dimension of which corresponds to the speed V, and a third dimension corresponds to the modeled value of the coefficient of adhesion. Although such a diagram is already in itself a tool for estimating the performance of a PN tire in a braking situation, the method of the invention may furthermore comprise a PREDICT predictive phase enabling a balance sheet to be obtained. more elaborate performance of this tire.

  To do this, this PREDICT phase can include including a DET IND operation for determining a global indicator INT (MU3D) of tire performance. This DET_IND operation consists for example of providing, as a performance indicator, the integral, with respect to the instantaneous speed V, and with respect to the sliding ratio g%, of the function MU3D on an integration domain formed instantaneous speeds and slip rates corresponding to a real or simulated SQC_FR braking sequence of a vehicle equipped with the corresponding PN tire. Preferably, the braking sequence SQC FR chosen to define the range of integration of the instantaneous speed V and the sliding ratio g% includes a modulation of the braking torque K of the type that a braking system with an anti-lock function of wheels or ABS.

  Typically, for a braking of 100 km / h until the complete stop of the vehicle, this indicator INT (MU3D) would be constituted by the integral of the function MU3D, calculated between 0 and 100 km / h for the variable V , and between 5% and 15% for the variable g% .35 By incorporating the MU3D function to the dynamic model of a vehicle, it is also possible to predict the braking distance of the modeled vehicle, equipped with tires having a coefficient of adhesion modeled by the function MU3D, and applying a braking sequence SQC FR is obtained from real measurements, or corresponding to a synthesized signal, or generated by a braking model of ABS type, that is to say equipped with an anti-lock wheel feature.

Claims (10)

CLAIMS.   1. A method for estimating the performance, in a braking situation, of a tire (PN) capable of equipping a vehicle, this method comprising an experimental phase (EXPERIM) including at least one step (EVAL MU) of evaluation of the grip coefficient () of the tire (PN) on the ground (S), characterized in that this evaluation step (EVAL MU) is performed for different values (Vi) of the relative displacement speed (V) of the axis (X) of the tire relative to the ground and for different values (gi%) of the slip rate (g%) of the tire on the ground, and in that said experimental phase (EXPERIM) further comprises a step model (MODEL) consisting in representing the coefficient of adhesion () of this tire, as it results from the evaluation step (EVAL MU), by a function (MU3D) of the displacement speed (V) relative to the tire axis (X) relative to the ground (S) and the slip rate (g%) of the tire on the ground.   Method according to claim 1, characterized in that the step (EVAL MU) of evaluating the coefficient of adhesion () of the tire (PN) comprises the simultaneous operations of rolling the tire (PN) on a surface of reference (S) assimilated to the ground, to apply the tire (PN) on the reference surface (S) with a pressing force (Pz) of known intensity by adjustment and / or by measurement, to be scrolled by relative to the axis (X) of the tire, the reference surface (S) at an instantaneous speed (V) of relative displacement, known by adjustment and / or measurement, and decreasing from a maximum value to a minimum value , to measure the instantaneous angular speed of rotation (w) of the tire (PN), and to periodically apply to the tire (PN) an instantaneous value braking torque (K) known by regulation and / or by measurement and capable of causing a sliding of the tire (PN) on the reference surface (S). 5   3. Method according to claim 2, characterized in that the coefficient of adhesion () is evaluated by the ratio (Fx / Fz), to the support force (Fz), the longitudinal force (Fx) corresponding to the torque braking force 10 (K).   4. A method according to any one of the preceding claims combined with claim 2, characterized in that the periodic application operation of the braking torque (K) is controlled to vary the slip rate (g%) of the tire (PN) relative to the reference surface (S) between a minimum value of preferably zero and a maximum value of at least 20%. 20   Method according to Claim 3, characterized in that the periodically applying operation of the braking torque (K) is regulated to periodically reach the value of the slip rate (g%) for which the adhesion value () is maximum. 25   6. Method according to any one of the preceding claims, characterized in that the step (EVAL MU) of evaluating the coefficient of adhesion () of the tire is carried out in a room with a substantially constant temperature.   7. A method as claimed in any one of the preceding claims, characterized in that the modeling step (MODEL) comprises an optimization operation (OPTIM) of searching for values to be assigned to predetermined parameters for a parametric expression. of said function (MU3D) of the speed (V) relative displacement of the axis (X) of the tire (PN) relative to the ground (S) and the slip rate (g%) of this tire on the ground corresponds to at best the coefficient of adhesion () as evaluated, during the evaluation step (EVAL MU), for the different values (Vi) of the speed (V) relative displacement of the axis (X) of the tire (PN) with respect to the ground (S) and for the different values (gi%) of the slip rate (g%) of the tire (PN) on the ground (S).   8. A method according to claim 7, characterized in that the optimization operation (OPTIM) uses the following parametric expression: Mu3D = (D * sin (C * atan (B * X1-E * (B * X1- atan (B * X1))))) + Sv, where D, B and X1 correspond to the following parametric expressions: D = DO + D1 * V + D2 * V2, B = BO + B1 * V + B2 * V2, X1 = (g% / 100) + Sh, and where V is the instantaneous speed of relative displacement of the tire axis (X) relative to the reference surface (S), and where g% is the slip rate of this tire relative to this area, defined by: g% = 100 * (V - Vp) / V where Vp is the peripheral speed of the tire. 35   9. Method according to any one of the preceding claims, characterized in that it further comprises a predictive phase (PREDICT) including an operation (DETIND) for determining a global indicator (INT (MU3D)) of the tire performance. , consisting in integrating, with respect to the instantaneous speed (V) relative displacement of the axis (X) of the tire (PN) and the ground (S), and with respect to the slip rate (g%) of this tire on the ground, the function (MU3D) representative of the coefficient of adherence of the tire, over a range of instantaneous speeds and slip rates corresponding to a real or simulated braking sequence (SQC FR).   Method according to Claim 9, characterized in that the braking sequence (SQC FR) chosen to define the integration domain of the instantaneous speed (V) and the slip ratio (g%) includes a modulation of the torque of braking (K) of the type encountered on a vehicle equipped with an anti-lock brake system or "ABS".